1. Field of the Invention
The present invention relates to a device for soldering printed circuit board assemblies and, more specifically, to an apparatus for measuring the height of a solder wave projected from the soldering vessel of an automatic soldering system and for maintaining the solder wave height at an optimum level to maximize the efficiency of the soldering process.
2. Background Art
Electronic products are often equipped with printed circuit boards (PCB) assemblies that have various components soldered and mounted onto the board. The automated fabrication of PCB assemblies has led to the development of automatic soldering systems to solder the PCB assemblies after additional components are inserted onto the board. The electrical components are mounted on the printed circuit board by inserting the leads of the components through apertures in the board from one side of the board. Projecting portions of certain leads that protrude through the board are then crimped over to secure the components on the board. The leads are then soldered to contact pads surrounding the apertures, and protective solder coatings are formed on conductor paths of the board, by passing the board assembly over a molten solder wave.
A molten solder surface tends to oxidize even when traveling in a molten solder wave and thus has an inherent high surface tension. Thus, as the printed circuit board assembly passes over the solder wave, unless a suitable fluxing agent is used, the molten solder in the solder wave tends not to separate between adjacent contact pads and conductor paths. This generates solder defects. As such, it is standard to spray a soldering flux on printed circuit board assemblies prior to their soldering.
An automatic soldering system may be constructed using conveyors that are installed, in spaced apart fashion, along the length of the automatic soldering apparatus. A printed circuit board having many additional components attached is transported by the conveyors during the soldering process.
A flux is applied by the automatic soldering system prior to soldering by transporting the printed circuit board so the bottom surface of the printed circuit board contacts the flux. The flux is unevenly applies over the bottom surface of the printed surface board using this method. Flux cleans the metal surfaces of the printed circuit board, prior to soldering, in the joint area by removing the oxide coating present. Additionally, flux also keeps the area clean by preventing the formation of oxide films and lowering the surface tension of the solder, thus increasing its wetting properties. A heater is installed next to the flux on the automatic soldering system. When the printed circuit board is transported over the heater by the conveyors, the printed circuit board is heated to a predetermined temperature. During the heating process, the flux that was applied to the bottom of the printed circuit board is melted. This causes the flux to evenly and thinly spread out over the bottom surface of the printed circuit board.
A soldering vessel is installed next to the heater in the automatic soldering system. The soldering vessel propels solder towards the bottom of the printed circuit board using air pressure. The propelled solder forms a solder wave that contacts the bottom of the printed circuit board. The air pressure that propels the solder may be generated by the revolutions of an impeller driven by a motor. The solder is applied to the bottom surface of the printed circuit board while the printed circuit board is transported over the soldering vessel by the conveyors.
The soldering efficiency of the automatic soldering apparatus depends on the height of the solder wave that is generated by the soldering vessel. It is important that the soldering process take place only on the underside of the object being soldered since the electrical components are attached on the other side of the printed circuit board assembly. The soldering wave must have a large enough height to make contact with the underside of the object being soldered and the soldering wave must also strike the underside of the printed circuit board with enough force to ensure a good soldering connection in hard to access locations. Thus accurate measurement of the solder wave is critical in an automated soldering process to ensure that good soldering connections are being made while avoiding soldering the top side of the printed circuit board assembly.
A solder wave height measuring apparatus may be constructed using a steel ruler to measure the height of a solder wave. The steel ruler is marked with graduations and is inserted into the melted solder contained in the soldering vessel. Then, the distance between the top of the soldering wave and the top surface of the soldering vessel is visually measured by reading the graduation that indicates the height H of the solder wave. The solder wave height is appropriately adjusted according to the difference between the desired solder wave height and the measured soldered wave height.
Some techniques for measuring the solder wave height and controlling automated solder operations are mentioned, for example, in U.S. Pat. No. 5,533,663 to Massini entitled Solder Wave Measurement Device, U.S. Pat. No. 5,617,988 to Willemen entitled Device for Measuring the Height of a Solder Wave, U.S. Pat. No. 4,363,434 to Flury entitled Continuous Soldering Installation, U.S. Pat. No. 4,291,575 to Frissora entitled Liquid Level Monitor, U.S. Pat. No. 4,529,116 to Gutbier entitled Methods of and Devices for Determining the Soldering Capability of a Solder Wave, U.S. Pat. No. 4,632,291 to Rahn entitled Automatic Wave Soldering Machine, U.S. Pat. No. 4,889,273 to Kondo entitled Soldering Apparatus, U.S. Pat. No. 5,148,961 to Humbert entitled Selective Wave Solder Apparatus, U.S. Pat. No. 5,388,468 entitled Solder Wave Parameters Analyzer, U.S. Pat. No. 5,598,345 to Tokura entitled Method and Apparatus for Inspecting Solder Portions, and U.S. Pat. No. 5,686,994 to Tokura entitled Appearance Inspection Apparatus and Appearance Inspection Method of Electronic Components.
Contemporary solder wave monitoring devices are technically complex, relatively expensive, and difficult to implement with low end automated manufacturing systems. I believe it may be possible to improve the art of solder wave height monitoring by providing a detecting device that has a relatively low cost, that is easy to operate, that uses few component parts, that readings can be visually made from without requiring operators to bend over into inconvenient positions, that can be easily maintained in an environment that does not have easy access to high technology components, and that is easy to operate, repair, and understand by less skilled workers.